1. Field of the Invention
The present invention generally relates to electromechanical transducers and, in particular, to transducers that directly convert an applied electrical signal into biaxial surface deformation.
2. Background Art
The background of the present invention includes biaxial surface deformation of generally solid bodies, associated with dilatation, piezoelectric deformation, and a subclass of mechanically induced strains. In the Applicant's U.S. Pat. No. 4,928,030 issued May 22, 1990, he describes electromechanical actuators having a responsive surface portion that is forcefully positioned in response to an applied electrical signal. The actuators combine transducer segments called lifters and tangenters that respectively translate the responsive surface perpendicular to, and in the plane of, the responsive surface. The mechanical stroke of the described actuator is the sum of the stroke contributions of the independently electrically stimulated transducer body segments. Separate embodiments of the lifter may use either the extension mode, the thickness mode, or the shear mode actions of the respective electrodeformable lifter body. Shear lifter action advantageously entails negligible strain at a bond between the lifter body and another rigid body because no biaxial deformation occurs at the bond. However, lifters using the extension and thickness deformations incur significant stress at bonds because these deformations are irrefutably coactive. The bond of an extension or thickness transducer segment to a rigid member relegates a portion of the segment's unbonded stroke and force to stress in and proximate the bond.
In Applicant's copending patent application, Ser. No. 01/108,643 filed May 31, 1991, he describes transducers that produce a surface twisting by shearing, the direction of twist being generally circular with respect to an axis normal to and passing through a point of the surface of a sheet of electrodeformable material, and the magnitude generally increasing monotonically from zero at the axis to a predetermined maximum value at the extremes of the sheet. Also taught are methods of making twisting transducers by uniformly radially sensitizing a sheet with subsequent adjustment to radially varying magnitude of responsivity by partial desensitization, by activating a sheet in a predetermined manner using nonuniform electric fields, and by applying a radially varying activating electric signal. Biaxial deformation transducers and methods of making same are not described.
U.S. Pat. No. 4,202,605 of Heinz issued May 13, 1980 describes an optical mirror that is deformed in a predetermined manner by the concerted action of a multitude of triaxial piezoelectric. transducers. Each transducer incorporates a piezoelectric body portion of each of the respective orthogonal directions. Each portion consists of many sheets of piezoelectric material bonded together with intervening insulating layers. Portions are similarly bonded together to constitute the transducer body. One of the body portions uses the thickness piezoelectric deformation that is irrevocably accompanied by a biaxial surface deformation. The biaxial deformation causes internal stress and strain in each bond with another transducer body portion and in the bond to a rigid support means, thereby sacrificing a portion of the otherwise available free-body stroke and force to bond stress. Electrically insulating layers are taught as a means to provide electrical independence of each layer of the transducer body, and of each motional segment of the body. Insulating layers reduce the fraction of the volume of the body of the transducer that contributes a useful forceful stroke. The insulating layers add to the length of the transducer body, thereby reducing the rigidity of the transducer in directions that tend to bend the body as a cantilever beam. Some of the characteristics of an ideal transducer not taught by Heinz are: maximum electromechanical efficiency gained through the use of no insulating layers; maximum rigidity by obviating insulating layers; and, full free-body forceful stroke of every body portion obtained by avoiding stresses in bonds due to frustrated biaxial surface deformation.
U.S. Pat. No. 3,558,351, issued Jan. 26, 1971 describes methods representative of those used to make shear electrodeformable materials by depositing volatilized material at an angle to the plane of deposition. The methods as taught do not produce deposited electrodeformable material with other than uniform magnitude and direction of responsivity within an individual material body.
U.S. Pat. No. 3,202,846 issued Aug. 24, 1965 is representative of the known methods of cutting an intrinsically electrodeformable material to obtain a desired electrodeformation, for example, shear, or shear combined with another mode of deformation. The benefit of intrinsically electrodeformable material is that sensitizing for electrodeformation by a temporary electrical connection is not required before application of permanent activating electrical connections. Another benefit of intrinsically electrodeformable materials is their general tendency to recover essentially full responsivity after cooling from a prescribed temperature above which responsivity is temporarily reduced or even disappears.
U.S. Pat. No. 4,523,121 issued Jun. 11, 1985 teaches methods of reducing operating stress in thickness mode piezoelectric transducers that have end sheets lying near the rigid bonding surface by progressively increasing sheet thickness in proportion to the proximity of the rigid bonding surface. When all sheets have the same applied electric potential, electric field intensity decreases in inverse proportion to sheet thickness. Reduced electric field intensity results in reduced piezoelectric deformation. An infinitely thick sheet bonded to the rigid surface will reduce the stress there to zero. In practice, there is generally insufficient space available for an infinitely thick sheet. In addition, the rigid bonding surface has finite elastic modulus that accommodates some of the stress by means of elastic shear compliance. However, progressively thicker sheets near the rigid bonding surface increase the size and therefore the axial elastic compliance of the actuator. Increased compliance decreases the forceful stroke otherwise available from the more compact actuator.
U.S. Pat. No. 4,649,313 issued Sep. 19, 1989 in the context of thickness-mode piezoelectric wafer stacks, teaches an intermediate piezoelectric thickness mode layer near the rigid bonding surface, the piezoelectric response of which is made less than that of the neighboring and main body layers. The forceful stroke of the intermediate layer is decreased in proportion to the decrease in piezoelectric responsivity. Although an intermediate layer with zero responsivity affixed to the rigid support means would affect zero operating stress at the bond, the intermediate layer would merely replace the rigid substrate at the bond to the transducer layer on the opposite side. Therefore, an intermediate value of responsivity of an intervening layer effectively divides up the stress between wafer interfaces but does not eliminate the stress. The intermediate layer still decreases the forceful stroke otherwise available from the same actuator having every layer with the greater piezoelectric responsiveness.
Known materials exhibit reversible plastic deformation when stressed below the value commonly referred to as the elastic limit. Very rigid and intrinsically accurate positioning transducers, typified by piezoelectric embodiments, also exhibit reversible plastic deformation. In general, the amount of deformation is related to the magnitude of the stress and to the time the stress obtains. An ideal transducer that stores relatively little energy of elastic strain, particularly in bonds between body portions and bonds to other rigid components of an apparatus retains the preponderance of its inherent transducing accuracy and stability with time.